eye opener, an

1

A startling or shocking revelation, as in The first sentence of his speech was a real eye opener. This expression alludes to widening one's eyes with surprise. [Mid-1800s]

2

A strong alcoholic drink taken early in the morning, as in After a late night and little sleep, he generally needed an eye opener to jolt him awake. This usage alludes to the alleged wakening effect of such a beverage. [Early 1800s]